The circuit structure of existing self-excited push-pull converters is mostly the self-excited oscillation push-pull transistor single-transformer DC converter invented by American G. H. Royer in 1955, which is also the beginning of the application of high-frequency conversion control circuit. Some other push-pull converters may use the self-excited push-pull double-transformer circuit, known as the self-oscillating Jensen circuit invented by American Jen Sen in 1957. These two circuits are collectively called the self-excited push-pull converter which has been described in Page 67 to 70 of “Principle and Design of Switching Power Supply” published by Publishing House of Electronics Industry, ISBN No. 7-121-00211-6. The main forms of the circuits described therein are the Royer circuit and Jensen circuit.
FIG. 1-1 shows a common application of a self-excited push-pull converter based on the Royer circuit structure, where the capacitor C1, parallel connected with bias resistor R1, can be omitted in many situations. Patent No. ZL 03273278.3 titled “A Self-excited Push-pull Converter” published on Aug. 25, 2004, provided a Royer circuit with a soft-start function (see FIG. 2) and thereby solved the problem of the impact caused by capacitance C1 on the push-pull transistors at the instant moment of switching-on.
FIG. 1-2 is also an application of the converter with a Royer circuit structure. What differs from FIG. 1-1 is that the bias resistor R1 is divided into Rlu and Rld in series, which is mostly used in higher operating voltage situations. Similarly, as the capacitor C1 (parallel connected with bias resistor R1) can be omitted in many situations, it is drawn with a dashed line in FIG. 1-2.
FIG. 3 shows a common Royer circuit with a simplified feedback winding and the return circuit of DC signals and the working point of transistors TR1 and TR2 are the same. However, when the circuit is in self-excited oscillation, transistor TR1 and TR2 are working differently. Patent application titled LCD BACKLIGHT DRIVER (Publication No.: US 2007182342 (Al), Publication Date: Aug. 9, 2007) showed a Royer circuit similar to that in FIG. 3 as the unit circuit. FIG. 4 is the original circuit form of FIG. 3. As its main characteristic, FIG. 4 uses two bias resistors Rla and Rlb respectively placed between each base of the push-pull transistors and the effective power supply end. FIG. 3 simplifies it by replacing these two bias resistors with just one and thus realizes a cost-effective solution based on FIG. 4. In patent application titled “Switching Power Supply Apparatus” (Publication No.: US 2006250822 (A1), Publication Date: Nov. 9, 2006), the bias resistors are used in the unit circuit similarly to that in FIG. 4.
FIG. 5 shows a familiar Royer circuit. Due to the inductance L1 in series with the power supply circuit and the capacitor CL parallel connected between the collectors of the push-pull transistors, the circuit output is close to the sine wave, which is commonly used in the electronic rectifier circuit of energy-saving lamps. Also, it may use the simplified method of feedback winding similar to the variation between FIG. 3 and FIG. 4.
The oscillation frequency of the Royer circuit is the function of supply voltage, which has been described in the 18th line of page 68 in the book “Principle and Design of Switching Power Supply” published by Publishing House of Electronics Industry, ISBN No. 7-121-00211-6. It is quoted below:
                    f        =                              Vs                          4              ⁢                                                          ⁢              BwSN                                ×                      10            4                    ⁢                                          ⁢          Hz                                    Formula        ⁢                                  ⁢        1            
In this formula, f refers to the oscillation frequency, Bw is the working magnetic induction (T), N stands for the number of coil turns and S means the effective cross-sectional area of the magnetic core.
The circuit structure in FIG. 1-1: Input filter capacitor C is connected between the input and ground to filter input voltage. The filtered input voltage is connected to the start-up circuit which is composed of bias resistor R1 and capacitor C1 in parallel. C1 can be omitted when the supply voltage input is relatively higher. The two ends of bias resistor R1 are connected with the input voltage and the center tap of primary coils NB1 and NB2 of the coupling transformer B, which provides positive feedback for the bases of two push-pull switching transistors TR1 and TR2, respectively. The emitters of TR1 and TR2 share a common ground and two collectors are respectively connected with the two ends of primary coils NB1 and NB2 of the coupling transformer with which the base is also connected, and the center tap of primary coils NB1 and NB2 is connected to the voltage input end. The secondary coil NS of coupling transformer B is connected with the output filter circuit in voltage output end.
The working principle can be briefly described as follows: referring to FIG. 1-1, the Royer circuit conducts push-pull oscillation relied on the feature of magnetic core saturation. At the moment when power is on, the parallel circuit of bias resistor R1 and capacitor C1 provides forward bias for the bases and emitters of transistor TR1 and TR2 through coil winding of NB1 and NB2, then two transistors are conducted. Because the properties of transistor cannot be made totally the same, one of them will be conducted first. On the assumption that TR2 is firstly conducted and produces collector current IC2, the upper side of corresponding coil winding of NP2 is positive and lower side negative. According to the dotted terminal relationship, the base coil winding of NB2 also shows an induced voltage positive in the upper and negative in the lower, which increases the base current of TR2. This is a process of positive feedback, thus enabling quick saturation and conduction of TR2. Similarly, the winding voltage of coil corresponding to transistor TR1 is positive in the upper and negative in the lower, which decreases the base current of TR1, causing it to cut off quickly and completely.
The current in the coil winding of NP2 (which is corresponding to TR2) and the magnetic induction produced by the current will increase linearly with time, but when the magnetic induction reaches the value of the core saturation Bm of coupling transformer B, the coil inductance then rapidly decreases, thereby causing a rapid increase in the current of TR2's collector at a speed much higher than that in the base current. Then TR2 is out of saturation and the voltage drop UCE from its collector to its emitter increases. Accordingly, the voltage of winding of transformer NP2 decreases by the same value and the induction voltage of winding of coil NB2 also decreases, causing a drop in the base voltage of TR2 and leading towards the cut-off of TR2. At this moment, the voltage in transformer coil will be reversed and the other transistor (i.e., TR1) will conduct. Now, the above described process will repeat itself to form the push-pull oscillation. The output waveform of winding Ns is shown in FIG. 6.
The characteristic of the above described process: conducting push-pull oscillation based on the properties of core saturation and the output waveform of the coupling transformer is approximately a square wave with a higher conversion efficiency. On the other hand, the circuit in FIG. 5 produces an output waveform close to the sine wave because of the existence of inductance L1 in the power supply circuit and the parallel connected capacitor CL between the two transistors' collectors.
FIG. 7 shows another structure similar to the Royer circuit, where the switch driving function and main power transformer are separated. It is the well-known self-oscillating Jensen circuit. The self-oscillation frequency and driving function of this circuit is realized with a magnetic saturation transformer, thus the main power transformer B1 works in an unsaturation state. In this circuit, only use one capacitor, CI or CIa. CIa is an equivalent to CI, but with CIa the circuit can realize soft starting. It should be noted that the circuit remains operable even if both C1 and CIa are removed.
For the Jensen circuit, even though B2 operates in magnetic saturation, due to its volume, it consumes a very small amount of energy and the overall efficiency of the circuit is high. Compared with the Royer circuit under the same conditions, the Jensen circuit has a comparatively more stable self-oscillation frequency when subject to working voltage, loads and temperatures fluctuations.
For that reason, the Jensen circuit is comparatively more widely used and has various forms, which are mainly reflected in different bias modes of R1 such as the one shown in FIG. 7.
FIGS. 1-7 (except FIG. 6) are all existing self-excited push-pull converters. Their common shortcomings are as follows:
1. Poor adaptability to working voltages.
In the no-load situation, with the increasing working voltage of the circuit, the input current which is equivalent to the quiescent current goes up and causes increased no-load loss of the circuit.
Table 1 lists the measurement result of conducted on the Royer circuit with the following setup: use the circuit shown in FIG. 1-1, set the input at DC 5V, the output at DC 5V, and the output current at 200 mA, that is, an output power of 1 W. The specification of main circuit components is: capacitor C is 1 uF, R1 is 1KΩ, capacitor C1 is 0.047 uF, transistors TR1 and TR2 have an amplification capacity of about 200 folds, and their collectors' maximum operating current is 1 A. The transformer's output circuit is shown in FIG. 8, a common full-wave rectifying circuit.
Throughout the testing process, no parameter of the circuit was readjusted and no part was replaced. For tests with the operating voltage at 12V or greater, the testing period was short because a longer period would cause damages on the circuit due to its large no-load loss.
TABLE 1InputNo-loadCircuitOperatingcurrent inloss ofInputOutputOutputconversionvoltageno-loadcircuitcurrentvoltagecurrentefficiency(V)(mA)(mW)(mA)(V)(mA)(%)39271832.76814573.1413522073.91216377.0518902545.00020078.78352802698.49420480.5126982825813.39316772.21590135022517.23812965.920180360025923.17013158.6
In the test, when the operating voltage was 5V, set the output current at 200 mA. At other operating voltages, the load was adjusted accordingly to make the output current as close to 200 mA as possible, but stop adjusting the load when the output voltage dropped by 5%.
Table 2 lists the measurement results of the Jensen circuit. The tests were conducted using the circuit shown in FIG. 7. The input was set at 5V DC and output 5V DC and 200 mA (i.e., output power 1 W). The specification of main circuit components is: capacitor C is 1 uF, resistor R1 is 1KΩ, capacitor Cla is 0.047 uF, and transistors TR1 and TR2 have an amplification capacity of about 200 folds and their collectors' maximum operating current is 1 A. The transformer's output circuit is shown in FIG. 8.
TABLE 2InputNo-loadCircuitOperatingcurrent inloss ofInputOutputOutputconversionvoltageno-loadcircuitcurrentvoltagecurrentefficiency(V)(mA)(mW)(mA)(V)(mA)(%)41248823.74263.272.1516801124.87185.674.5830.4243.21198.08182.770.212135162022012.73780.939.015200300025616.06054.122.6
Formula for calculating the conversion efficiency of the circuit is:
                    η        =                                            Vout              ×              Iout                                      Vin              ×              Iin                                ×          100          ⁢          %                                    Formula        ⁢                                  ⁢        2            Note: Vin=operating voltage, i.e. input voltage, Iin=input current; Vout=output voltage, Iout=output current.
As shown in Table 1, for the circuit with an operating voltage of 5V, when it was working at 8V, the self-loss reached 280 mW, which is barely acceptable for a micropower DC/DC converter. At 12V, the self-loss reached 828 mW, and at 20V 3600 mW (i.e. 3.6 W), under which the circuit would be damaged if operating for over 3 seconds. Thus, the circuit conversion efficiency decreases with increasing operating voltages. The Jensen circuit showed the same problem at elevated operating voltages: the no-load operating current increases too fast, causing higher no-load loss and lower conversion efficiency.
2. Poor performance in surge handling. Based on the above described reasons, when a surge occurs in input voltage, the circuit can be easily damaged, mainly on the transistors.
3. Designs of self-excited push-pull converter in other operating voltages all present the same aforementioned flaws.